I had what i thought was a knock in my 400 sbc so i pulled it and the oil pan....there doesn't appear to be anything amiss. during the process i found that 1 foot was broken off the torque converter and the center of the flexplate was about 97% busted out in a very jagged pattern that allowed it to continue to start and work. my question is, could that have created the hollow knock sound during startup and low rpm idling in drive?
also, i scraped the old oilpan gasket off (real mess) and even though i was careful, i know i got some crap down in the block.....any good way to clean it out and still button it back up?
thanks....

Flexplate cracks can cause significant low speed noise. So yes you found it.

Take a paper towel soaked in parts cleaning solvent and wipe up your mess the best you can. Don't turn the crankshaft until you have removed most all the mess. Then I would recommend rotating the crank a small amount to see if you've uncovered any more. What I would then do is assemble the engine and run it a short time, then drain the oil and change the filter.